Re: NanoVNA Under The Covers
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The supplied dummy load does give a reproducible 70dB return loss. It is a problem that unless this can be verified independently it is a software problem with the 3 point calibration procedure. I do not know enough about the nano vna architecture to confirm if it has this dynamic range. I do know that I have a return loss bridge and a power meter that can read 70dB. I just need to find some quality dummy loads to confirm firstly that the supplied dummy does have such a low return loss. If it does not then it's obviously the nano vna calibration procedure or hardware that is giving this over optimistic value. If it does the the nano vna is good.Sent from my Samsung Galaxy smartphone.
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-------- Original message --------From: alan victor <avictor73@...> Date: 04/08/2019 22:52 (GMT+01:00) To: [email protected] Subject: Re: [nanovna-users] NanoVNA Under The Covers I suspected that is what you did and sorry there is a problem with that measurement method.There is measurement validation and there is measurement verification.I believe you did a verification which after correction would show an excellent return loss to the extent that removal and re insertion of the 50 ohm load is identical and no errors were made during the cal process.Validation will require an independent set of cal standards. I think you hit upon this in your reply. Not sure. In any case, those standards should be well defined and not necessarily assumed to be ideal. They should be as precise as possible, for example,? built with a model definition that is defined and is then used by the VNA for correction of its own inherent errors. There is a degree of uncertainty in this so called correction. Nothing is perfectly corrected. With that said, then an independent measurement of the quality of your 50 ohm standards may occur. Again, these standards under test were NOT part of the calibration sequence.If I recall, measurement uncertainty for a typical instrument would result at best in a 50 dB return loss. Don't hold me to this number. I would have to back track through some old notes. However, my point is,? you are treading on an absolute measurement that is better than the uncertainty level. That is an issue.Regards, Alan________________________________From: [email protected] <[email protected]> on behalf of tuckvk3cca <tuckvk3cca@...>Sent: Sunday, August 4, 2019 7:00 PMTo: [email protected] <[email protected]>Subject: Re: [nanovna-users] NanoVNA Under The CoversYes I just use the 3 point calibration from the dummies provided. The 50 ohm load did show -70dB. I then use another sma dummy load I had and it show -40 dB or so. I am not saying I believe my results, that is why ai want to buy a few better ones to counter check. I do have a RL bridge that is good to -60dB up to 500 MHz at least. You can measure down to -78dB if you have an AD8307 power meter which I have. That chip can go down to -90dB in principle.Sent from my Samsung Galaxy smartphone.-------- Original message --------From: jimcking@... Date: 04/08/2019? 19:16? (GMT+01:00) To: [email protected] Subject: Re: [nanovna-users] NanoVNA Under The Covers [Edited Message Follows]I read -70 dB also on two different NanoVnas.? Ch0 LogMag @ 50 KHz using the 50 Ohm load that came with the NanoVna.? This is the same 50 Ohm that was used in the calibration.? The calibration procedure assumes the load to be perfect so return loss shows -70 dB.? It goes up with Frequency but is always better than -50 dB.Jim K.
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